1. Field of the Invention
The present invention relates in general to a high density read only memory (ROM) and a method of making the same. More particularly, the present invention relates to a ROM device and a method of making the same using a metal bit line and a metal word line.
2. Description of the Related Art
Read only memories (ROMs) are widely used in mini-computers, microprocessors, and other related digital hardware. ROMs can be used to store system data, for example, a basic input/output system (BIOS) in a personal computer. The manufacturing of a ROM is complicated and time consuming. Therefore, partially completed ROMs are typically produced and stored to reduce delivery time. Once the customer's programming data is received, the partially completed ROMs are programmed to complete the finished product.
Most ROM devices have similar structures, and differ only in the data which is stored during the programming step. Typically, once the integrated circuit (IC) factory receives a programming order from a customer, a mask is produced to perform the programming. This allows production to be rapidly finished, so that the manufacturing time is reduced, thus providing better service to the customers. Therefore, ROMs that are post-programmed with the aid of a mask are commonly used in the art.
In general, channel transistors are used as the memory cells in the ROM. In the programming process, specific channel regions of the channel transistors are selectively implanted with impurities to modify the threshold voltages of the transistors. This controls the conductivity of the memory cell. For a detailed description, reference is made to FIGS. 1A through 1C, in which a conventional ROM device is illustrated. FIG. 1A is a top view of a portion of the known ROM device. FIG. 1B is a front view of a portion of the known ROM device. FIG. 1C is a cross-sectional view of the known ROM device.
The conventional ROM device includes a substrate 10, a plurality of bit lines (BL) 11, an oxide layer 12, and a plurality of word lines 13. Substrate 10 comprises, for example, a P-type substrate. The bit lines 11, oxide layer 12, and the word lines 13 are formed on the substrate 10. As shown in FIG. 1A, the region defined by the dotted line forms a memory cell 14. The data (in binary form such as "0" or "1") is stored in the memory cell by implanting the channel region 16 with impurities.
As shown in FIG. 1C, an N-type dopant, for example, arsenic ions, is doped on the substrate 10 to form bit lines 11, which are arranged with the same distance between each other. Channel region 16 is formed between adjacent bit lines 11. Next, oxide layer 12 is formed on a surface of the bit lines 11 and channel regions 16 using an oxidation process. A conductive layer of, for example, heavily doped polysilicon, is formed over the substrate 10 using a photolithography and etching process to form word lines 13, which cross the bit lines 11. Then, channel transistors are formed to complete the partially manufactured conventional ROM.
To program the partially manufactured ROM, a series of continuous programming steps are performed. A mask 15 is formed over the partially completed ROM, leaving the channel region 16 which is to be encoded exposed. Then, a P-type dopant, for example boron, is implanted to form the code. However, the type of the implanted dopant is selected depending on the characteristics of the transistor.
In the conventional ROM described above, the metal-oxide-semiconductor transistor is difficult to minimize, thus, the device is difficult to integrate. Further, in the manufacturing process of the conventional ROM, the bit lines are formed by doping N.sup.+ ions on the substrate. Because the resistance of the bit lines is about 100 ohms per square, which is much higher than the resistance of typical metal conductors, operating current cannot be increased. Further, the lower breakdown voltage of the channel transistor will limit the operating voltage at decoding. These limitations may result in an output error.